A. Field of the Invention
The invention relates generally to a perpendicular magnetic recording medium mounted on various magnetic recording apparatuses and a method for manufacturing such a medium.
B. Description of the Related Art
In technologies for achieving high density magnetic recording, perpendicular magnetic recording is drawing attention as a substitute for conventional longitudinal magnetic recording.
Alloy materials of CoCrPt, CoCrTa, and the like have been used for a magnetic recording layer material of a perpendicular magnetic recording medium employing perpendicular magnetic recording. In these alloy materials, the nonmagnetic substance chromium segregates into a grain boundary and magnetically isolates each crystal grain for a magnetic recording medium so that it exhibits required characteristics such as high coercivity (Hc). The chromium segregation into the grain boundary has been promoted in the in-plane medium by controlling the deposition process, including heating and substrate bias voltage application.
In a perpendicular magnetic recording medium, however, the heating or substrate bias voltage application as conducted in the in-plane medium can segregate only a small amount of chromium, resulting in high media noise. To solve this problem, a granular medium has been proposed in which magnetic isolation of crystal grains is promoted by segregation of oxides or nitrides into the grain boundary. In a granular film of CoCrPt—SiO2, for example, SiO2 segregates surrounding the CoCrPt crystal grain. The granular film does not use phase separation (magnetic phase separation) of an alloy material, but features addition of an amorphous material such as an oxide or nitride that hardly makes a solid solution with an alloy material. The granular medium has been confirmed to reduce media noise in comparison with a conventional medium comprising a magnetic recording layer of CoCr alloy material. Therefore, the granular medium is deemed promising for magnetic recording media.
In order to further raise the recording density of a magnetic recording medium, magnetic isolation of the magnetic crystal grains in the magnetic recording layer must be promoted and magnetization reversal unit must be decreased. For media using a continuous film such as a granular medium, magnetic isolation of magnetic crystal grains in the magnetic recording layer is enhanced by promoting segregation of the nonmagnetic substance at the grain boundary. For discrete media such as patterned media the known methods attempt to decrease magnetization reversal unit by processing the magnetic recording layer using a technique such as etching employed in a semiconductor process. It has been difficult, however, to promote segregation in the granular magnetic recording layer by a process that is suitable for mass production.
To achieve a high density recording medium requires increased signal output at the high recording density in addition to noise reduction by promoting segregation. Perpendicular magnetic recording media are known to produce stable bits in high density recording. At the same time this means that taking out magnetic flux in high density recording is difficult. While a medium structure is required that achieves low noise and high signal output even at high recording density, such a medium structure has never been proposed.
There have been various problems in achieving the desired result. Because deposition of a granular film with substrate heating raises problems of mixing a nonmagnetic phase with an alloy phase and/or oxidation or nitridation of cobalt, the film must be deposited without substrate heating. However, film deposition without substrate heating cannot achieve sufficient isolation between the alloy phase and the nonmagnetic phase, which raises a problem of insufficient noise reduction. To solve this problem, heat treatment has been conducted at a high temperature from 400° C. to 600° C. after depositing the layers up to the magnetic recording layer (or to a protective film) without heating. Thus, proposals have been made to obtain a granular medium that has enough isolation of magnetic particles from nonmagnetic matrix and allows high density recording by carrying out heat treatment at a temperature not lower than 400° C. for 5 to 60 minutes in Japanese Unexamined Patent Application Publication No. 2000-306228 and at a temperature from 250° C. to 500° C. for 0.1 to 10 hours in Japanese Unexamined Patent Application Publication No. 2000-311329, for example. Heat treatment at such a high temperature and for relatively long time is not suitable for mass production.
Japanese Unexamined Patent Application Publication No. 2001-202611 proposes a technique to promote magnetic isolation of crystal grains in the magnetic recording layer, in which grain size of the magnetic layer is continuously varied along the film thickness direction. This document teaches varying the grain size of the magnetic layer along the film thickness direction by varying a ratio of magnetic material to nonmagnetic material in the magnetic recording layer. Specifically, separate targets are prepared for the magnetic material and the nonmagnetic material and power supplied in the sputtering process is changed in several (typically five) steps to vary mixing ratio of the magnetic material to the nonmagnetic material and to control the grain size. It is unfortunately very difficult to obtain a film that is homogeneous over a disk surface using an arrangement that places two targets in equipment for mass production. Therefore, this technique is not suitable for mass production.
There are further known examples: patterned media (Japanese Unexamined Patent Application Publication No. H10-233015, for example) and use of a self-organized film in which fine particles are arranged two-dimensionally. (Japanese Unexamined Patent Application Publication No. H10-320772 and Japanese Unexamined Patent Application Publication No. 2002-334414, for example.) However, none of these techniques is suitable for mass production.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.